This first phase of the combustion is very abrupt: the fuel vapour, premixed with hot air (under the pressure and temperature conditions required for self-ignition), ignites on mass. The reaction speed is very high and each partially vaporised droplet has very rapidly consumed all of the oxygen present in the air which is mixed with the vapour. In such a short time, since the mixture is not homogeneous, the unmixed air does not have time to sustain combustion, taking into account its distance from the centre (the droplet) of the combustion. The reaction therefore stops very rapidly, or at least slows down due to the rarefaction of the available oxygen. This mass combustion phase (uncontrolled combustion) is called the "pre-mix combustion".
The air and fuel movements which are preestablished or induced by the injection of the highly-pressurised fuel, or brought about by the expansion of the gases heated by the abrupt chemical reaction during this first phase of combustion allow the exothermic reaction to follow. The latter then develops in a controlled fashion, by virtue of the mass transfers, by diffusion, from the zones rich in fuel to the zones which are poor in fuel, that is to say towards the zones where the oxygen content is high. This phase of combustion by diffusion is called "progressive combustion". It is much slower and continues at the rate of mixing sustained by the relative movements of air and fuel in the working chamber.
Diagrams a, b, c and d of the appended FIG. 1 illustrate the phenomena which have just been described. These diagrams have, as common abscissa, the angle .alpha. of the crankshaft with respect to a defined angular position, the common ordinate axis Y corresponding to TDC of the piston. Diagram "a" shows the variation in pressure of the cylinder of the engine, in solid line (curve A) when there is combustion, and in broken line (curve B) when there is no combustion. Diagram "b" shows the variation in the position of the injection needle (curve C), which needle forms part of the abovementioned means for introducing pressurised fuel, and it shows up the ignition delay .tau. which is expressed in angular duration between the angular position of the crank shaft corresponding to the start of the introduction of fuel and that corresponding to the start of ignition, detected by the release of heat. In diagram "c", the instantaneous release of heat has been brought onto the ordinate axis, the first area D corresponding to the pre-mix combustion and the second area E corresponding to the progressive combustion. Finally, in diagram "d" the accumulative quantity of heat, in percentages of the total heat released during one cycle, has been brought onto the ordinate axis, the curve F thus obtained consequently being tangential to the 100% ordinate axis.
The longer the ignition delay, the greater the quantity of fuel injected before ignition, which leads to the following drawbacks:
The constructors of diesel engines have therefore tried to reduce the ignition delay (for example by retarding the instant at which the fuel is introduced) whilst seeking to cool the fresh air inlet into the cylinder or cylinders, so as to increase its density and so as not to exceed, as far as possible, the cycle temperatures above which the oxides of nitrogen tend to be produced in excessive quantity, which tends to increase the ignition delay. The solutions which they have proposed up until now have not given entire satisfaction, particularly from the point of view of efficiency and emissions of particulates and smoke from the exhaust.